1. Field of the Invention
The present invention relates to a driving circuit for a flat panel display and, more particularly, to an active matrix organic light emitting diode AMOLED driving circuit using current feedback that ensures the uniformity of brightness in pixels of a flat panel display and shortens the time required for inputting accurate current to the respective pixels in the driving circuit.
2. Description of the Related Art
An Organic Light Emitting Diode (OLED) is an element for a flat panel display that has attracted attention recently because it has several merits, that is, it has excellent viewing angle and contrast ratio, it is thin and lightweight, it has low power consumption and can be fabricated at a lower cost.
OLED is an element from which light emission is regulated based on the current applied thereto, and is classified into a passive matrix method and an active matrix method in view of the method of driving the OLED.
In the active matrix method, the voltage for controlling the current applied to the OLED is charged in a capacitor and the charged voltage is kept until a new signal is applied to a subsequent frame.
A conventional pixel circuit and a driving circuit using an OLED having such characteristics will now be described with reference to U.S. Pat. Nos. 5,748,160 and 6,433,488.
FIG. 1 is a basic pixel circuit, depicted in the former, which constitutes a panel having the form of an M×N matrix.
M scan lines SCAN and N data lines Vdata exist in the panel, wherein N pixel circuits in a single row are coupled in parallel to a single scan line SCAN, and M pixel circuits are coupled in parallel to a single data line Vdata.
A driving transistor T1 implemented using a thin film transistor TFT controls the current applied to an OLED. Since the driving transistor T1 and the OLED are connected in series to each other, the current flowing in the driving transistor T1 is identical to that flowing in the OLED.
The current of the driving transistor T1 can be controlled by a voltage data line Vdata suitable to the current-voltage characteristic curve of the driving transistor T1.
Besides, the magnitude of the current of the driving transistor T1 is controlled by the input voltage applied from a switching transistor T2, and the input voltage is charged in a storing capacitor Cs, and then maintained until a subsequent frame starts.
However, in the conventional pixel circuit, the amount of current applied through the same input voltage may vary due to differences between the threshold voltages of the driving transistors, each having one TFT per pixel, thus causing non-uniformity of brightness in the respective pixels.
Accordingly, a current driving method has been proposed to solve such non-uniformity of driving currents due to the differences between the characteristics including the threshold voltages in the respective pixels.
In the voltage driving method depicted in FIG. 1, a voltage for controlling the current to be applied to the OLED is input, whereas, in the current driving method, the current to be applied to the OLED is itself input.
Accordingly, the desired current can be applied to the OLED regardless of differences between the threshold voltages of the respective driving transistors and variation in current mobility.
FIG. 2 shows a driving circuit employing the current driving method using current feedback according to U.S. Pat. No. 6,433,488.
A driving part, except for the pixel circuit in FIG. 2, exists in the respective columns of a panel, to which M pixel circuits are coupled in parallel. The selection of pixel circuits to be driven among the M pixel circuits is made in response to a scan signal SCAN.
A transistor T1 is a driving transistor and transistors T2, T3 and T4 are switching transistors. When the scan signal SCAN is high, transistor T4 is turned off, whereas transistors T2 and T3 are turned on, thus forming a loop comprising transistors T1 and T2, a current comparator, a transistor T3 and an organic light emitting diode.
Here, the current flowing in the driving transistor T1 is a current IOLED applied from a current source IOLED, and a current to be newly input is a current IREF from a current source IREF. Accordingly, the current comparator compares the two currents to apply a control voltage VFB to a gate node of the transistor T1.
The control voltage VFB applied to the gate node of the transistor T1 varies IOLED, which consequently converges to IREF, and corresponding voltage is charged in a capacitor Cs.
However, since a plurality of pixel circuits is connected to one driving circuit of FIG. 2, considerable parasitic capacitance is generated in the data line and in the input of the current comparator.
The parasitic capacitance makes it difficult to secure the stability of the feedback loop and increases the overall response time of the circuit, thus affecting the time required to input current to the pixel circuits.
Particularly, in the case of a larger sized panel, since a much greater number of pixel circuits is coupled to one driving circuit, which results in increased parasitic capacitance, it is very difficult to secure the stability of the feedback loop and the current input speed.
Besides, as the number of pixel circuits per driving circuit is increased, the useful time for updating information in a pixel circuit is reduced. Accordingly, securing the current input speed becomes the most important issue because the current should be input within the reduced time.
More particularly, the current range of the parasitic capacitance of the current driving part (the parasitic capacitance of the anode node of the OLED) is within IOLED, and the current amount is no more than several nA to several μA. Accordingly, if the driving current is not supplemented in this node, it causes considerable difficulty in securing the current input speed.